Niobium pentoxide (Nb2O5) has been extensively reported for applications of electrochemical energy storage, memristors, solar cells, light emitting diodes (LEDs), and electrochromic devices.The thermal properties of Nb2O5 play a critical role in device performance of these applications.However, very few studies on the thermal properties of Nb2O5 have been reported and a fundamental understanding of heat transport in Nb2O5 is still lacking. The present work closes this gap and provides the first study of thermal conductivity of amorphous Nb2O5 thin films. Ultralow thermal conductivity is observed without any size effect in films as thin as 48 nm, which indicates that propagons contribute negligibly to the thermal conductivity and that the thermal transport is dominated by diffusons. Density-function-theory (DFT) simulations combined with a diffusonmediated minimum-thermal-conductivity model confirms this finding. Additionally, the measured thermal conductivity is lower than the amorphous limit (Cahill model), which proves that the diffuson model works better than the Cahill model to describe the thermal conduction mechanism in the amorphous Nb2O5 thin films. Additionally, the thermal conductivity does not change significantly with oxygen vacancy concentration. This stable and low thermal conductivity facilitates excellent performance for applications such as memristors.

Memristors are a promising technology to surpass the limitations of the current silicon complementary metal-oxide-semiconductor architecture via the realization of neuromorphic computing. Here, we demonstrate intercalation-based non-volatile lithium niobite (Li1 – xNbO2) memristors for highly scalable, efficient, and dense neuromorphic circuitry. Volatile, semi-volatile, and non-volatile operation is achieved using a single material, where each operational mode provides a timescale that enables short-term, medium-term, and long-term memory in conjunction with computation-in-memory. The two-terminal non-volatile devices exhibit conductance changes of up to ∼2000% and have inherent non-binary operations proportional to flux linkage, allowing for analog neuromorphic functions mimicking synaptic weight updates. It is shown that Li1 – xNbO2 devices are highly scalable due to the intercalation-based mechanism, with non-volatile operation requiring a mere 150 mV for a 4 μm2 device, the lowest reported operating voltage for an inorganic non-volatile memristor. The programming voltage scales linearly with device size, projecting millivolt operation and attojoule energy consumption for nanoscale devices.

Neuromorphic computation using nanoscale adaptive oxide devices or memristors is a very promising alternative to the conventional digital computing framework. Oxides of transition metals, such as hafnium (HfOx), have been proven to be excellent candidate materials for these devices, because they show non-volatile memory and analog switching characteristics. This work presents a comprehensive study of the transport phenomena in HfOx based memristors and involves the development of a fully coupled electrothermal and mass transport model that is validated with electrical and thermal metrology experiments. The fundamental transport mechanisms in HfOx devices were analyzed together with the local and temporal variation of voltage, current, and temperature. The effect of thermal conductivity of substrate materials on the filament temperature, voltage ramp rate, and set/reset characteristics was investigated. These analyses provide insight into the switching mechanisms of these oxides and allow for the prediction of the effect of device architecture on switching behavior.

Metal-Nb2O5−x-metal memdiodes exhibiting rectification, hysteresis, and capacitance are demonstrated for applications in neuromorphic circuitry. These devices do not require any post-fabrication treatments such as filament creation by electroforming that would impede circuit scalability. Instead these devices operate due to Poole-Frenkel defect controlled transport where the high defect density is inherent to the Nb2O5−x deposition rather than post-fabrication treatments. Temperature dependent measurements reveal that the dominant trap energy is 0.22 eV suggesting it results from the oxygen deficiencies in the amorphous Nb2O5−x. Rectification occurs due to a transition from thermionic emission to tunneling current and is present even in thick devices (>100 nm) due to charge trapping which controls the tunneling distance. The turn-on voltage is linearly proportional to the Schottky barrier height and, in contrast to traditional metal-insulator-metal diodes, is logarithmically proportional to the device thickness. Hysteresis in the I–V curve occurs due to the current limited filling of traps.

High-quality epitaxial growth of thin film lithium niobate (LiNbO3) is highly desirable for optical and acoustic device applications. Despite decades of research, current state-of-the-art epitaxial techniques are limited by either the material quality or growth rates needed for practical devices. In this paper, we provide a short summary of the primary challenges of lithium niobate epitaxy followed by a brief historical review of lithium niobate epitaxy for prevalent epitaxial techniques. Available figures of merit for crystalline quality and optical transmission losses are given for each growth method. The highest crystalline quality lithium niobate thin film was recently grown by halide-based molecular beam epitaxy and is comparable to bulk lithium niobate crystals. However, these high-quality crystals are grown at slow rates that limit many practical applications. Given the many challenges that lithium niobate epitaxy imposes and the wide variety of methods that have unsuccessfully attempted to surmount these barriers, new approaches to lithium niobate epitaxy are required to meet the need for simultaneously high crystalline quality and sufficient thickness for devices not currently practical by existing techniques.